In situ pressure leaching method

ABSTRACT

A nuclear chimney located below the water table in a primary ore deposit serves as an in situ pressure vessel in which ore can be oxidized and a desired element contained in the ore subsequently recovered in a leach solution. After the nuclear chimney has flooded with water under substantial hydrostatic pressure, oxidizing gas is percolated through the contained ore and water mixture to dissolve and oxidize the ore minerals. Ammonia may be added with the oxidizing gas to render the solution basic. The solution is pumped from the chimney and may be recirculated with or without heating until the desired concentration is obtained. The oxidizing gas may also be heated prior to its introduction into the chimney.

United States Patent 1151 3,640,579 Lewis Feb. 8, 1972 [54] IN SITU PRESSURE LEACHING METHOD Primary Examiner-Emest R. Purser Attorney-Roland A. Anderson [72] Inventor: Arthur E. Lewis, 844 Santa Rita Avenue,

Los Altos, Calif. 94022 57 ABSTRACT Filed: P 1970 A nuclear chimney located below the water table in a primary ore deposit serves as an in situ pressure vessel in which ore can [21] Appl' 32679 be oxidized and a desired element contained in the ore subsequently recovered in a leach solution. After the nuclear [52] U5. CL ..299/4, 75/103, 299/5 himney h flooded wi h wa r un er subs nti l hydr s atic 51 1 1m. 01 ..E21b 43/28 pressure, oxidizing gas is Percolated through the contained ore 58 Field of Search ..299 4,5;75/101, 103 and water mixture to diswlve and oxidize the ore minerals- Ammonia may be added with the oxidizing gas to render the [56] References Cited solution basic. The solution is pumped from the chimney and may be recirculated with or without heating until the desired UNITED STATES PATENTS concentration is obtained. The oxidizing gas may also be heated prior to its introduction into the chimney. 2,563,623 8/1951 Scott ..299/5 X 3,545,964 1 2/1970 Hansen et al. ..299/5 X 10 Claims, 1 DrawingFigure 5 0 LU T l O N 21 PROCE SSI N G PLANT i COMPRESSED 1B GAS SOURCE PAIENEpm'amz 3.640.579

SOLUTION 21 PROCESSI us PLANT COMPRESSED 18 GAS SOURCE 19 1? PUMP INVENTOR. Arthur E. Lewis ATTORNEY.

IN SITU PRESSURE LEACHING METHOD BACKGROUND OF THE INVENTION The invention disclosed herein was made in the course of or under contract W-7405-Eng.48 with the United States Atomic Energy Commission.

Various primary metallic ore deposits, such as ores of copper, silver, gold, molybdenum, and similar valuable elements, consist of minerals containing the desired element unaltered by weathering or associated processes. Many of such primary deposits have been upgraded by processes of oxidation and supergene enrichment to concentrations that make them economically feasible to mine. The enrichment process is a near-surface process, although subsequent geological events may result in the burial or removal by erosion of the ore deposits.

Most ore deposits have been developed by mining the enriched and oxidized zone thereof. Only where the primary ore is rich enough has it been economical to mine at depth or, with large and efficient earth-moving equipment, to mine large deposits in open-pit mines. Leaching techniques have been applied economically to the oxidized parts of ore deposits or to those that can be oxidized by alternate solution and weathering or bacterial oxidation. Because of the low permeability of such ore deposits, this has not been done in place, with the exception of some leaching operations conducted on caved material in mine workings. In general, primary ores such as primary sulfides, e.g., chalcopyrite, have not been economically leachable by present technology, and as a result the economic processing of these ores has been accomplished by sulfide concentration and smelting. Hence, large deposits of such ores remain undeveloped because they lie too deep to mine economically, their size is too small to allow the economics of large-scale mining operations, the grade is too low, or various combinations of these factors. Reserves of copper and other elements would be considerably increased if an economical process of obtaining such elements from deep primary ore deposits were available.

With a view towards economic recovery of elements from deep ore deposits, various nuclear explosive mining methods have been heretofore proposed. In this regard, it has been contemplated to employ a contained nuclear explosion to break up a deep ore deposit and form a rubble chimney therein. A leach solution is then percolated through the chimney to oxidize the broken ore and cause the desired element to go into solution. The solution is pumped to the surface and the desired element recovered therefrom by conventional processes. However, it has been found that at the low pressures, such as atmospheric pressure, which typically exist in the chimney, various ores, e.g., chalcopyrite and most other primary sulfide minerals, are not effectively dissolved by the leach solution, or the rate of solution is so slow that any such process is impractical. It is known that the oxidation process and rate of solution are accelerated by the addition of an oxidizing agent, such as oxygen, chlorine, ferric sulfate, or the like, to the solution and ore. However, many of such agents are too expensive or corrosive to use. Moreover, although oxygen is relatively inexpensive and noncorrosive, it is not very soluble at atmospheric pressure. Consequently an oxidizing process utilizing oxygen as the oxidizing agent cannot be conducted in situ at the low pressures existent in a conventional chimney and would entail the removal of the broken ore and conduct of the leaching process in a high-pressure vessel, such as an autoclave. Recovery of a desired element by such a process would of course be economically impractical because of the cost of ore removal and relatively small scale of the leaching process.

SUMMARY OF THE INVENTION The present invention relates to the economic recovery of valuable elements from their primary ores, and is more particularly directed to the chemical mining of primary ores by in situ pressure leaching.

In accordance with the present invention a mining method is provided which employs well-known nuclear technology to form a rubble chimney by underground nuclear detonation. However, whereas it has been the usual approach to attempt to avoid fracturing into the water table inasmuch as filling of the chimney with water has been considered undesirable in most cases, according to the present invention a flooded nuclear chimney formed beneath the water table is employed to good advantage to provide properties which are uniquely beneficial to the economic chemical mining of primary ores. More particularly, in accordance with the method of the present invention a contained nuclear explosive is detonated in a primary ore deposit well below the water table to provide a mass of broken ore in a flooded chimney subjected to substantial hydrostatic pressure, the chimney thereby serving as an in situ pressure vessel suitable for pressure leaching of the ore. The hydrostatic pressure increases the solubility of an oxidizing gas introduced to the chimney sufi'lciently to allow the primary ore compounds to be oxidized and dissolved in a relatively short time. The leach solution is subsequently pumped to the surface and the desired element recovered therefrom by conventional processes, such as replacement, electrolysis, hydrogen reduction, ion exchange, etc.

BRIEF DESCRIPTION OF THE DRAWING The invention is illustrated in the accompanying drawing wherein the single FIGURE is a cross section of a geological formation with a nuclear chimney formed in a primary ore deposit and adapted to the conduct of a leaching method in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawing in detail, there is shown a primary ore deposit 11 which includes compounds of a valuable element which is desired to be obtained. For example, in the case of copper the ore may comprise primary copper sulfides such as chalcopyrite, bornite, chalcocite, etc. In accordance with the particularly salient aspects of the present invention, such a deposit 11 is first located which exists well beneath the water table 12. Thereafter, a nuclear chimney 13 of broken ore is formed in the deposit entirely beneath the water table, preferably at a substantial depth thereunder. Formation of the chimney is accomplished in general accordance with wellestablished nuclear technology by emplacing and detonating a nuclear explosive having a predetermined yield, at a predetermined depth commensurate with a contained explosion in the ore deposit productive of a rubble chimney of broken ore having a predetermined height above the detonation point selected to not exceed the water table 12. It is particularly important to note that although fracturing into the water table is usually to be avoided in the detonation of most contained underground nuclear explosions in order to prevent flooding of the resulting rubble chimney, in accordance with the present mining method, the nuclear explosive is deliberately emplaced and detonated beneath the water table. As a result, the chimney I3 is flooded by the natural inflow of water under the hydrostatic head of its vertical displacement below the water table 12. The broken ore in the chimney is thus subjected to hydrostatic pressures which are dependent upon the prevailing water head. In this regard, the hydrostatic pressure acting on a given point is approximately 435 p.s.i. or 30 bars for each 1 ,000 feet it is below the water table.

The hydrostatic pressure acting on the nuclear chimney I3 is utilized to good advantage in accordance with the method of the present invention to promote leaching of the broken ore contained therein. More particularly, the chimney I3 is employed as an in situ pressure vessel for accelerating the dissolution of the contained broken primary ore minerals. In this regard, an oxidizing gas is introduced into the chimney at high pressure and thereby percolatcs through the contained broken ore and water mixture which has a substantially infinite permeability compared to that of the surrounding unbroken wall rock. By virtue of the high pressure, the solubility of the gas is increased such that it readily dissolves and oxidizes the ore minerals to form an acid leach solution in the reaction. The acid solution aids the oxidation of the ore minerals by the gas and the desired ore element goes into solution at a relatively rapid rate. The leach solution is subsequently extracted from the chimney and the desired element recovered from the solution as by means of replacement reaction, electrolysis, or other conventional processes, or the solution is recirculated through the chimney until the concentration of the desired element is sufficiently high to optimize profitability in the subsequent recovery process.

The foregoing leaching and element recovery process is preferably accomplished by drilling at least one shaft 14 to communicate with the bottom of the chimney l3 and at least one shaft 16 to communicate with the top thereof. The oxidizing gas is then introduced to the bottom of the chimney via the shaft 14, as by means of a pipe 17 extending therethrough in spaced relation to the shaft wall and connected at its upper end to a suitable compressed gas source 18, such as a gas supply and compressor, at the ground surface. The gas thus rises upward through the chimney and any excess not reacting will rise to the ground surface through the shaft 16. Extraction of the leach solution from the chimney is preferably accomplished by pumping same to the ground surface as by means of a pump 19 communicated with the chimney via shaft 16. The pump delivers the solution to a solution processing plant 21 which functions to recover the desired element from the solution and/or to recirculate the solution to the chimney through the clearance space existing between the pipe 17 and wall of shaft 14. In this manner the solution can be recirculated until the concentration of the desired element dissolved therein is high enough for profitable recovery.

The leaching method outlined hereinbefore is particularly applicable to the economic recovery of copper from deep deposits of primary copper sulfides, such as chalcopyrite (Cu Fe S In this case, oxygen is employed as the oxidizing gas introduced to the chimney containing broken ore and water. Because of the high pressure in the chimney, the oxygen dissolves in the water in sufficient quantity to oxidize some of the pyrite (Fe S and produce sulfuric acid (11 S and ferrous sulfate (Fe 80,) in the reaction. The ferrous sulfate and sulfuric acid in the presence of the abundant oxygen in turn react to produce ferric sulfate [Fe (SO.,) Ferric sulfate in the presence of sulfuric acid forms a solution that will dissolve chalcopyrite, and most other copper minerals. The copper goes into solution as a sulfate and may be thus recovered by replacement with metallic iron which causes the copper to precipitate, or by electrolysis which causes the copper to precipitate at the cathode.

It will be appreciated that the leaching method of the present invention may be likewise applied to the recovery of other elements from their primary ores in a substantially similar manner to that detailed for copper. For primary ores of elements similar to copper, oxygen is a suitable oxidizing gas. For ores of various other elements, such as gold and molybdenum, chlorine may be a suitable oxidizing gas for employment in the leaching process. in addition with some elements, such as molybdenum, it may be desirable to utilize a basic, rather than an acid leaching system in which case a gas such as ammonia may be introduced to the chimney in addition to the oxygen or other oxidizing gas. Moreover, in some instances it may be desirable to increase the reaction rate by increasing temperature. To this end the oxidizing gas and/or the recirculated solution may be heated prior to its introduction to the chimney. In addition, the reaction rate may be increased by the addition to the solution of chloride ions in the form of common salt, or the like. However it should be noted that the oxidation process is exothermic and heat is deposited in the chimney by the explosion such that the in situ temperature is already relatively high without additional heating in the aforementioned manner. Consequently, by virtue of the tremendous quantity of ore that can be processed in the chimney at one time, somewhat slower reaction rates carried out at the in situ temperatures can be economically tolerated.

Considering now a particular example of the leaching method for purely illustrative purposes, consider the case of a deep deposit having an ore grade of 0.5 percent copper, bulking factor of 0.15, and rock density of 2.7 g./cc. in an area where the water table is at 50 meters. From well established nuclear explosive technology, it can be shown that a nuclear explosive with a yield of kt. emplaced and detonated in the ore deposit at a burial depth of 750 meters is productive of a cavity with radius 41 meter and volume of 2.9 l0 cubic meters. Assuming a chimney wall slope of 3 from the vertical, it can be calculated that the chimney has a height of 250 meters above the shot point and contains 1.6X10 cubic meters of broken ore. With the chimney thus positioned and proportioned, the hydrostatic pressure acting thereon varies from 1,000 psi. at the bottom of the chimney to 650 psi. at the top. Oxygen at a pressure of about 1,000 psi. is introduced to the chimney at a rate of 5,000 ft. /hr. for a time sufficient to cause about 50 percent of the copper in the ore to go into solution. It can be shown that the copper in solution is then substantially 38 g./l., or a total of 1 1X10 grams. Assuming a copper price of 0.1 g., the copper is hence worth $11 million. The cost of developing this amount of copper is estimated at about $3million for detonation of the nuclear explosive, $1 million for drilling costs, and $7,000/year for delivery of the oxygen to the chimney. The cost of the solution processing plant is of relatively low order. Consequently, it will be appreciated that the worth of the copper far exceeds the cost of developing same in accordance with the pressure leaching method of the present invention.

There is thus provided by the present invention an economic chemical mining method for the development of deep deposits of primary ores which have been economically unfeasible to mine by existing methods. Large increases in the reserves of low-cost copper and other elements may be obtained without the necessity of finding new ore deposits, because the type of deposit minable by the present method exists below many known deposits and can be developed easily. Also many new deposits are currently being discovered at depths which may be effectively mined by the present method. Moreover with the method 'no dumps or open pits are produced which would disfigure the landscape; waste solutions are circulated back into the chimney, thereby avoiding any stream pollution; and there is no atmospheric pollution common to other recovery operations such as smelting.

Although the invention has been hereinbefore described and illustrated in the accompanying drawing with respect to specific steps of the method thereof, it will be appreciated that various modifications and changes may be made therein without departing from the true spirit and scope of the invention, and thus it is not intended to limit the invention except by the terms of the following claims.

1 claim:

1. A leaching method for the recovery of a desired element from primary ore containing same comprising the steps of locating a primary ore deposit containing a desired element existing beneath the water table, detonating a contained nuclear explosive in said ore deposit to form a flooded rubble chimney of broken ore at a substantial depth beneath said water table subjected to the hydrostatic pressure head of the vertical displacement of said chimney therefrom, introducing oxidizing gas to said chimney at substantially the pressure of said hydrostatic head with said gas being highly soluble at said pressure in the water contained in said chimney and oxidizing said ore to form an acid leach solution effective in the presence of the dissolved gas to dissolve said ore and cause said desired element to go into solution, extracting said solution from said chimney, and recovering said desired element from said solution.

2. A leaching method according to claim 1, further defined by heating said gas prior to its introduction to said chimney to increase the temperature therein and accelerate the oxidation reaction.

3. A leaching method according to claim 1, further defined by introducing ammonia together with said oxidizing gas to said chimney for dissolution in said solution to render it basic.

4. A leaching method according to claim 1, further defined by said desired element being copper, said ore being primary copper sulfides, and said gas being oxygen 5. A leaching method according to claim 1, further defined by recirculating said solution through said chimney until a predetermined concentration of said desired element is contained therein prior to recovery of said element therefrom.

6. A leaching method according to claim 5, further defined by heating said solution during the recirculation thereof to increase the temperature in said chimney and accelerate the oxidation reaction.

7. A leaching method according to claim 1, further defined by the introduction of oxidizing gas to said chimney comprising drilling at least one first shaft to communicate with the bottom of said chimney, and delivering said gas to the bottom of said chimney via each first shaft; and the extraction of said solution from said chimney comprising drilling at least one second shaft to communicate with the top of said chimney, and pumping said solution from said chimney to the ground surface via each second shaft.

8. A leaching method according to claim 7, further defined by delivering said solution pumped to the ground surface to the bottom of said chimney via each first shaft to recirculate said solution through said chimney until a predetermined concentration of said desired element is contained therein prior to recovery of said element therefrom.

9. A leaching method according to claim 8, further defined by heating said gas prior to its introduction to said chimney and heating said solution during the recirculation thereof to thereby increase the temperature in said chimney and accelerate the oxidation reaction.

10. A leaching method according to claim 7, further defined by delivering ammonia with said oxidizing gas to the bottom of said chimney for dissolution in said solution to render it basic. 

2. A leaching method according to claim 1, further defined by heating said gas prior to its introduction to said chimney to increase the temperature therein and accelerate the oxidation reaction.
 3. A leaching method according to claim 1, further defined by introducing ammonia together with said oxidizing gas to said chimney for dissolution in said solution to render it basic.
 4. A leaching method according to claim 1, further defined by said desired element being copper, said ore being primary copper sulfides, and said gas being oxygen.
 5. A leaching method according to claim 1, further defined by recirculating said solution through said chimney until a predetermined concentration of said desired element is contained therein prior to recovery of said element therefrom.
 6. A leaching method according to claim 5, further defined by heating said solution during the recirculation thereof to increase the temperature in said chimney and accelerate the oxidation reaction.
 7. A leaching method according to claim 1, further defined by the introduction of oxidizing gas to said chimney comprising drilling at least one first shaft to communicate with the bottom of said chimney, and delivering said gas to the bottom of said chimney via each first shaft; and the extraction of said solution from said chimney comprising drilling at least one second shaft to communicate with the top of said chimney, and pumping said solution from said chimney to the ground surface via each second shaft.
 8. A leaching method according to claim 7, further defined by delivering said solution pumped to the ground surface to the bottom of said chimney via each first shaft to recirculate said solution through said chimney until a predetermined concentration of said desired element is contained therein prior to recovery of said element therefrom.
 9. A leaching method according to claim 8, further defined by heating said gas prior to its introduction to said chimney and heating said solution during the recirculation thereof to thereby increase the temperature in said chimney and accelerate the oxidation reaction.
 10. A leaching method according to claim 7, further defined by delivering ammonia with said oxidizing gas to the bottom of said chimney for dissolution in said solution to render it basic. 